1. Field of the Invention
The process of the present invention is directed to purifying raw chromium metal which has been obtained from an electrolytic, aluminothermic, or other pyrometallurgical processes. The process of the present invention treats the raw chromium metal with hydrogen gas and a vacuum at elevated temperatures to reduce the carbon (C), nitrogen (N), oxygen (O) and sulfur (S) content of the chromium metal. The purified chromium metal is suitable for metallurgical and electronic applications which demand chromium metal with low gaseous impurities.
2. Description of Related Art
Raw chromium metal is prepared through either an electrolytic process, an aluminothermic process, or other pyrometallurgical processes. Electrolytically prepared chromium metal is obtained as plates, and has a gaseous impurity content typically of 0.006 wt % C, 0.5 wt % O, 0.03 wt % N, and 0.03 wt % S. Aluminothermically produced chromium metal is produced as lumpy masses and, subsequently, ground into smaller sizes. The contents of gaseous impurities in the aluminothermically produced chromium metal vary depending on the raw materials mix order and on the sample positions in the reactors. A typical impurity analysis of the aluminothermically produced and degasifying-grade chromium metal is 0.03 wt % C, 0.5 wt % O, 0.05 wt % N, and 0.02 wt % S. Other pyrometallurgical processes which produce a raw chromium metal are the carbothermic reduction of chromium oxide or chromium oxyhydroxide under a vacuum. Again, the chemistry of the raw chromium metal obtained by these two processes vary depending on the mix order and processing conditions. Usually, the impurity analyses of carbon and oxygen show a greater variance than the other processes. The impurity contents of carbon and oxygen for the degasifying-grade chromium metal made by carbothermic reduction are in the range of 0.01 to 0.3 wt % C and 0.03 to 0.35 wt % O when chromium oxyhydroxide is used, and 0.89 to 1.76 wt % C and 1.18 to 1.71 wt % O when chromium oxide is used. In all cases, the raw chromium metal typically has a chromium content of about 99.1 wt %.
Some critical metallurgical applications for chromium metal, such as turbine engine parts, demand a low content of the gaseous impurities in chromium metal. The contents of the gaseous impurities in the chromium metals prepared by electrolytic, aluminothermical, or other pyrometallurgical processes are too high for critical metallurgical applications, and raw chromium metals need to be refined to lower these impurities to the level less than 0.003 wt % C, 0.03 wt % O, 0.002 wt % N, and 0.001 wt % S.
The conventional refining process of raw chromium metal uses powdered chromium metal in order to minimize the reaction time. The chromium metal powders are, however, agglomerated into pellets or briquettes for efficient handling during the refining process. Binders are usually added in order to provide a green strength to the pellets or briquettes. Other reactants are also added to the powder at the time of briquetting to achieve the intended refining reactions. For example, carbon is added to remove the oxygen; and tin, nickel, copper, or mercury is added to remove the sulfur.
The conventional refining process treats the pellets or briquettes at 1100.degree. C. to 1500.degree. C. under a vacuum in order to control the residual contents of C, O, N, and S. See U.S. Pat. No. 5,092,921.
One of the problems associated with the conventional refining process is that the final chemistry of the refined chromium metal depends on the precise control of the stoichiometric relationships of the added reactants, quality of the blending, and the conditions of the refining reactions. Often, problems occur in that the added reactants in the agglomeration suffers an inevitable weighing error, the blending of the ingredient mixes is insufficient, and/or the processing variable for the refining reactions are not controlled well. As a result, the chemistry of the final products can be inconsistent.
A variation of the conventional process is to forego the addition of desulfurizing agents. See U.S. Pat. No. 4,504,310 and GB 2,255,349A. Such a process, however, does not control the sulfur content.
There is a need for a commercially viable process which controls the gaseous elements of C, O, N, and S together, and produces consistent results.